In normal hearing, sound causes mechanical vibrations that stimulate the hair cells of the cochlea to produce electrical impulses that travel down the auditory nerve where they are perceived by the brain as sound. If for some reason these hair cells are destroyed or not present within the cochlea, as is the case with individuals with severe or profound hearing loss, the nerve cells do not receive this electrical stimulation, therefore no sound is perceived. A cochlear implant attempts to replace this lost function by providing artificial electrical stimulation of the surviving auditory nerve. Cochlear implants have been in clinical use for many years. Such devices use an array of implanted electrodes to provide electrical stimuli to the cochlea. The electrical stimuli are determined by a processor responsive to speech and sound signals in the environment of the user.
Historically, prior to around 1994, the majority of speech processors used in conjunction with a cochlear implant employed speech processing strategies that can be described as Feature Extraction Strategies. In such strategies, the associated implant hardware attempts to identify the speech features present in the detected sound signal and encodes such features as patterns of electrical stimulation. Feature extraction strategies have the advantage that the hardware required to perform the feature extraction is relatively simple and consumes a relatively low amount of power.
With improvements in silicon chip technology and an increased knowledge of the safety of electrical stimulation, a new approach in sound processing became possible. This approach had the ability to provide a full range of spectral information of the speech signal without the need for the hardware to fit the signal into a preconceived mould, giving the patient the opportunity to listen to the particular information of interest, within background noise, providing a more realistic approach to speech processing. Such sound processors use band-pass filters to separate acoustic signals into frequency bands or spectral components with relatively little overlap of the bands, with the electrodes being stimulated in a tonotopic fashion according to the energy in those bands. Usually they present a smoothed (low-pass-filtered) representation of the amplitude from each band to a single electrode.
Despite considerable practical success with each of the existing schemes, the user perceptions of existing devices indicate that there are significant outstanding problems. Three fundamental problems of sound perception reported by cochlear implant users are poor frequency resolution and discrimination, poor perception of speech in noise at low signal-to-noise ratios, and poor perception of musical sounds.
It is an object of the present invention to provide an alternative speech processor and processing method, in order to further improve the practical performance of the cochlear implant system.